Periodic corrugations are often useful in semiconductor devices. In particular, distributed feedback (DFB) lasers incorporate a periodic grating in the laser structure which provides spatial variations in the refractive index. When a DFB laser is electrically excited constructive interference of diffracted light gives rise to a stable single wavelength output. In semiconductor devices it is also often useful to form a channel to define an optical waveguide. J. T. Andrews et al. in U.S. patent application No. b 80,171 (RCA) entitled "DISTRIBUTED FEEDBACK LASER", filed concurrently herewith, disclose a DFB laser comprising a semiconductor body having opposed end faces and having a channel which varies in width. The variation in channel width is periodic along the length of the channel. As the desired length of each period is often less than one micrometer (.mu.m) conventional photolithographic techniques to define the channel are generally unavailable.
Well-known holographic techniques have been used to form gratings with a period less than 1 .mu.m in length. The gratings formed generally comprise a plurality of parallel periodic elements. As these gratings are formed on a planar surface of a structure they have been considered inapplicable in producing a channel in the structure which periodically varies in width. Thus, it would be desirable to have a method for producing a variable width channel.